This invention relates to the release of pressure from oxidized ore slurry in an autoclave circuit. In particular, the invention relates to the design of a nozzle system through which ore slurry passes into slurry flash tanks.
Autoclave circuits are used to recover gold from refractory sulfidic ores. Ore leaving an autoclave is typically passed to a series of flash tanks where pressure is let down and steam is flashed off to cool the slurry, and reduce it to atmospheric pressure for further processing. Steam from each flash tank is recycled and contacted with autoclave feed slurry in a complementary splash condenser, operated at substantially the same pressure as the flash tank, for preheating the autoclave ore feed slurry. In one particular system the pressure from the autoclave slurry discharge is let down in two stages. In the first stage, pressure is let down from about 420 psig to about 120 psig. In the second stage, pressure is let down from about 120 psig to atmospheric. This second pressure drop corresponds to a much greater volume expansion than in the first stage.
Heretofore this second pressure drop from about 120 psig to atmospheric has been accomplished by use of a nozzle system comprising a straight tubular choke extending from outside the flash tank to inside the flash tank. The choke was surrounded by a ceramic lined blast tube extending further into to flash tank, as shown in FIG. 4. As the volume of the slurry expands rapidly upon passage through the choke, the blast tube was violently impacted with steam entrained with ore slurry. Catastrophic failure of the blast tube, resulting in ore slurry damaging and even breaching the low pressure flash tank, has occurred. The typical life of such nozzle systems, and in particular of the blast tubes, has been relatively short, for example, six weeks, depending on operating parameters, ore characteristics, and many other factors. Rebuilding and/or replacing such nozzle assemblies is expensive in terms of capital costs and in terms of process downtime.
Among the several objects of the invention, therefore, are the provision of an extended life nozzle system for a low pressure slurry flash tank; the provision of an improved apparatus for preheating gold ore slurry prior to pressure oxidation and for reducing the pressure of pressure oxidized gold ore slurry after pressure oxidation; and the provision of an improved process for reducing the pressure of pressure oxidized gold slurry.
Briefly, therefore, the invention is directed to a wear-resistant flash tank pressure let down nozzle for use in passing an ore slurry into an ore slurry flash tank to release steam from the slurry and reduce the pressure of the slurry. The nozzle has an inlet end, a discharge end, and a tubular passageway extending therebetween for passage of the slurry from a location outside the flash tank in fluid flow communication with the inlet end. The slurry passageway has a choke comprising a zone of the passageway in which its cross sectional area is smallest, the passageway flaring with respect to the axis thereof toward the discharge end to define an expansion cone. The cross-sectional area of the choke and the exit diameter of the expansion cone being selected to establish a relationship between pressure upstream of the nozzle and pressure in the flash tank so that shock waves inside the flash tank are weaker than shock waves inside a reference flash tank having identical dimensions and configuration and operating under identical conditions except having a reference pressure let down nozzle consisting of a straight choke.
The invention is also directed to an ore slurry flash tank apparatus for receiving and holding pressure oxidized ore slurry for reducing the pressure of pressure oxidized gold ore slurry. The apparatus has a vessel having a bottom, a top, and side walls, and a wear-resistant flash tank pressure let down nozzle. The nozzle has an inlet end, a discharge end, and a tubular passageway extending therebetween for passage of the slurry from a location outside the flash tank in fluid flow communication with the inlet end. The slurry passageway has a choke comprising a zone of the passageway in which its cross sectional area is smallest, the passageway flaring with respect to the axis thereof toward the discharge end to define an expansion cone. The cross-sectional area of the choke and the exit diameter of the expansion cone are selected to establish a relationship between pressure upstream of the nozzle and pressure in the flash tank so that shock waves inside the flash tank are weaker than shock waves inside a reference flash tank having identical dimensions and configuration and operating under identical conditions except having a reference pressure let down nozzle consisting of a straight choke.
In another aspect, the invention is directed to an apparatus for preheating gold ore slurry prior to pressure oxidation and for reducing the pressure of pressure oxidized gold ore slurry after pressure oxidation. There is a flash tank for receiving a volume of pressure oxidized gold ore slurry, the flash tank comprising a vessel having a bottom, a top, and side walls, and a nozzle on the top of the vessel for passing ore slurry into the vessel. The nozzle has an inlet end, a discharge end, and a tubular passageway extending therebetween for passage of the slurry from a location outside the flash tank in fluid flow communication with the inlet end. The slurry passageway has a choke comprising a zone of the passageway in which its cross sectional area is smallest, the passageway flaring with respect to the axis thereof toward the discharge end to define an expansion cone. The cross-sectional area of the choke and the exit diameter of the expansion cone are selected to establish a relationship between pressure upstream of the nozzle and pressure in the flash tank so that shock waves inside the flash tank are weaker than shock waves inside a reference flash tank having identical dimensions and configuration and operating under identical conditions except having a reference pressure let down nozzle consisting of a straight choke. There is a steam outlet for passing steam out of the flash tank, and a splash condenser for contacting ore slurry with steam prior to pressure oxidation of the ore slurry in order to preheat the ore slurry, the splash condenser having a steam inlet. There is also conduit for transferring steam from the steam outlet of the flash tank to the splash condenser.
The invention is further directed to a process for reducing the pressure of pressure oxidized ore slurry from above about 100 psig to about atmospheric. Slurry is passed through a nozzle into a flash tank, the nozzle disposed on the top of the flash tank and comprising a receiving end and a discharge end, and a slurry passageway extending through the nozzle from the receiving end to the discharge end for passage of the slurry into the flash tank from a location outside the flash tank. The slurry passageway flares outwardly at an angle of about 15xc2x0 toward the discharge end to gradually reduce the pressure of the slurry and to direct the slurry such that it impacts a volume of slurry in the bottom of the flash tank.
The invention is also directed to an ore slurry flash tank apparatus for receiving and holding pressure oxidized ore slurry for reducing the pressure of pressure oxidized gold ore slurry. There is a vessel having a bottom, a top, and side walls, and a nozzle on the top of the vessel for passing ore slurry into the vessel. The nozzle has an inlet end, a discharge end, and a tubular passageway extending therebetween for passage of the slurry from a location outside the flash tank in fluid flow communication with the inlet end. The slurry passageway comprises a choke comprising a zone of the passageway in which its cross sectional area is smallest, the passageway flaring with respect to the axis thereof toward the discharge end to define an expansion cone. The nozzle has a choke diameter of between about 3xc2xd and about 4xc2xd inches, an expansion cone exit diameter of between about 7 and about 7xc2xd inches, and an expansion cone length of between about 5xc2xe inches and about 6xc2xe inches, to establish a relationship between pressure upstream of the nozzle and pressure in the flash tank so that shock waves inside the flash tank are weaker than shock waves inside a reference flash tank operating under identical conditions except having a reference pressure let down nozzle comprising a straight choke.
In another aspect, the invention is directed to an ore slurry flash tank apparatus for receiving and holding pressure oxidized ore slurry for reducing the pressure of pressure oxidized gold ore slurry from between about 100 psig and about 140 psig to about atmospheric. There is a vessel having a bottom, a top, and side walls, and a nozzle on the top of the vessel for passing ore slurry into the vessel. The nozzle has a tubular passageway extending therebetween for passage of the slurry from a location outside the flash tank in fluid flow communication with the inlet end, the slurry passageway comprising a choke comprising a zone of the passageway in which its cross sectional area is smallest. The passageway flares with respect to the axis thereof toward the discharge end to define an expansion cone. The nozzle has a choke diameter of between about 3.8 and about 4.1 inches corresponding to the smallest cross-section in the slurry passageway, the straight section having a length of between about 9xc2xd and about 10xc2xd inches, an expansion cone exit diameter of between about 7.1 and about 7.4 inches, an expansion cone length of between about 6 inches and about 6.2 inches, and an expansion cone length of between about 14xc2x0 and about 16xc2x0, to establish a relationship between pressure upstream of the nozzle and pressure in the flash tank so that shock waves inside the flash tank are weaker than shock waves inside a reference flash tank operating under identical conditions except having a reference pressure let down nozzle comprising a straight choke.
Other objects and features of the invention will be in part apparent and in part pointed out hereinafter.